Can a Christian who has converted to Islam in order to get married convert back to Christianity?
The question is twofold. I therefore split it into a) and b). Re a): A Christian (I assume the question refers to a Catholic), who has converted to Islam is not only allowed to re-convert to Catholicism, he is required to do so. As long as he does not do he is eo ipso excommunicated (cf. can. 1364 § 1). Re b): is, from a canonical point of view, the marriage of a person who has left the Church and then married in any other form, e.g. in a civil ceremony or in an Islamic ceremony, valid? According to can. 1117 CIC/1983 it was valid. As this canon was changed by the Pope on 19. Dec. 2009 such marriages are now no longer valid. Now the old principle applies again: Once a Catholic, always a Catholic.